I had a question about the difference between air cylinders vs hydraulic cylinders. Hydraulic cylinders are easy to find, cheap as chips, many different sizes and configurations, rated to 3,000 psi usually. Air cylinders are a lot harder to find, a lot more expensive, and I only need 150 psi for the bus maximum. Is it possible to take a hydraulic cylinder and use it with air pressure? What are the key internal differences to think about? I am thinking of two applications off the top of my head - MCI style clutch assist, which is single acting, and door open/close which is double acting.

Air volume moves much faster than hydraulic oil and the seals on a hydraulic cylinder are fabricated with oil in mind. If you run them dry, they won't last long. I know that first hand from working on heavy equipment. Of course, that was many years ago and seal technology has probably changed. Will

I'm looking into the clutch assist thing too. I think air is going to be the way to go. I thought about using the power steering pump for the hydraulic assist cylinder on the clutch. The air assist is probably cheaper in the long run since you don't need expensive hydraulic hoses, control valves, and return to sump setup. Fluid flow regulators are more expensive. I would also think that fluid viscosity (cold to hot) would affect the assist quite a bit. The air assist would be more consistent.

Why not just go to a truck wrecking yard and buy the master cylinder and slave cylinder off a truck or buy the setup off a wrecked Prevost they have used it for years then you don't have all the design time and a proven unit

To be clear, I am asking if I can substitute a hydraulic cylinder that would normally use oil in an application that would use air. I can get a 2" bore 8" stroke hydraulic cylinder with the fittings I need down the street for almost free (just as an example), while an air cylinder I would have to spec out and special order from the internet. So I wondered - can I get one of the hydraulic cylinders and just use it with air instead of pressurized oil?

I don't know what stroke I need for the door yet, I haven't designed the system (custom retrofit deal). Probably the 2" bore 8" stroke would work, developing around 350 pounds of push maximum, probably use a bleed to control speed and a regulator to control push. Double acting so it both opens and closes. Skinner valves so it is electrically controlled. That sort of thing...

I wouldn't use a hydraulic cylinder for air,the big thing is volume vs pressure and sealing issues with air vs hydraulic the hydro cylinder is big and bulky and ugly....unless you need it

FYI a skool bus door opener (air) is a 5-3/8 stroke (atleast the 2 I have) with cleavice pin mounts and both ends are adjustible about 3/8 or 1/2 in and they allow a closed position to over 50* opening using the stock retro fitted mounts.

should be a $10 JY find

More is possible, if you optimize the angles etc.

As for strength?.. you cant push the door open when it's closed (i converted mine from folding bi-fold; to 1 pc swing open)

Hydraulic cylinders are built heavier to withstand higher pressures possible with hydraulic fluid. If you add an inline pneumatic lubricator, a hyd cylinder should be fine with air. Air cylinders usually have a very low "break away" force, so you should check the hyd cylinder to ensure it is acceptable. (Break away force = psi required to overcome the static friction of the seals.)

The real test is in connecting the hyd cylinder to compressed air & seeing that it moves as expected. Connect air to one port only & see if any leaks out the other port, if not, then you are good to go.

Let us know how it works out.

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In a piston-in-cylinder actuator, the force applied equals the pressure on the piston multiplied by the area of the piston. Off hand I'd say that a hydraulic actuator used at normal compressed air system pressure would cost much more than a comparable air cylinder. The hydraulic cylinder would be designed for thousands of psi, not hundreds.

That same Brill I had had an air operated clutch. It used a Bendix pedal, same as the brake treadle, and used a Rotochamber attached to a homemade mounting bracket at the bellhousing, connecting to the clutch arm and only had an air line from pedal to chamber, and supply to pedal. Worked fine with low pedal pressure. I would have used the same setup on the 4104 but it already had an air assist clutch.